When members for use in automobiles, home appliances, or buildings, for example, are to be produced, blanks (materials) are subjected to plastic working such as press forming to be formed into a predetermined shape. When producing the blanks in large volume, shearing for example is employed to cut a metal sheet into a predetermined shape.
FIG. 1 schematically illustrates how a metal sheet is cut by shearing. As illustrated in FIG. 1(a), when a metal sheet 1 is to be sheared, firstly the metal sheet 1 is placed on a die 2. Thereafter, as illustrated in FIG. 1(b), a punch 3 is moved toward the surface of the metal sheet 1 in a direction approximately perpendicular thereto (direction indicated by an arrow D) to cut the metal sheet 1.
FIG. 2 is a schematic cross-sectional view of an exemplary sheared edge of a metal sheet that has been cut by shearing. As illustrated in FIG. 2, a sheared edge 4 of the metal sheet 1 includes, for example, a shear droop portion 4a, a sheared surface 4b, and a fractured surface 4c. The sheared surface is significantly plastically deformed as a result of the shearing. In the example illustrated in FIG. 2, a burr 5 has been formed on the back side of the metal sheet 1 as a result of the shearing.
As described above, sheared edges include a sheared surface, which is significantly plastically deformed as a result of shearing. Thus, sheared edges cannot easily stretch and deform compared with worked surfaces formed by machining and grinding, and therefore sheared edges are more likely to have stretch flange cracking (cracking that occurs in the worked surface when the worked surface stretches during press forming, which follows the process of shearing, machining, or another process). In the following, stretch flange cracking will be described with reference to the drawings.
FIG. 3 presents diagrams for illustrating stretch flanging. FIG. 3(a) is a perspective view of a metal sheet before being subjected to stretch flanging, and FIGS. 3(b) and 3(c) are perspective views of the metal sheet after being subjected to the stretch flanging.
Referring to FIG. 3(a), the metal sheet 6 has been cut by shearing and a sheared edge 6a has been formed along the outer perimeter edge. The outer perimeter edge of the metal sheet 6 includes a recess 6b, which has an approximately L-shaped perimeter edge in plan view. The perimeter edge of the recess 6b includes a straight portion 6c, a curved portion 6d, and a straight portion 6e. In FIG. 3(a), a length X1, a length Y1, and a length Z1 represent the lengths of the straight portion 6c, the curved portion 6d, and the straight portion 6e, respectively.
Referring to FIGS. 3(a) and 3(b), in the case where stretch flanging is applied to a perimeter edge area of the recess 6b in such a manner as to cause out-of-plane deformation, the lengths X1, Z1 of the straight portion 6c and straight portion 6e do not change, whereas the length of the curved portion 6d changes to a length Y2, which is greater than the length Y1. That is, the sheared edge 6a stretches and deforms in the curved portion 6d. This may result in the occurrence of stretch flange cracking in the curved portion 6d. 
Referring to FIGS. 3(a) and 3(c), in the case where stretch flanging is applied to a perimeter edge area of the recess 6b in such a manner as to cause in-plane deformation, the lengths X1, 21 of the straight portion 6c and the straight portion 6e do not change (or substantially do not change), whereas the length of the curved portion 6d changes to a length Y3, which is greater than the length Y1. That is, the sheared edge 6a stretches and deforms in the curved portion 6d. This may result in the occurrence of stretch flange cracking in the curved portion 6d. 
The occurrence of stretch flange cracking as described above poses problems particularly when producing home appliance parts or automotive parts, which are of various types, by press forming. In recent years, there has been a need for further weight reduction of parts such as those mentioned above, and therefore thin steel sheets having a strength greater than or equal to that of 780 MPa class steel sheets are frequently used. Thus, suppression of the occurrence of stretch flange cracking is desired particularly when high strength steel sheets such as those mentioned above are subjected to press forming. However, it is known that stretch flange cracking occurs even in a low strength steel sheet, and therefore prevention of stretch flange cracking is necessary regardless of the strength of the steel sheet. Thus, many techniques have been proposed heretofore for suppressing the occurrence of stretch flange cracking in a sheared edge.
For example, Patent Document 1 discloses a punching tool in which the punch includes a projecting bending blade at the tip of the cutting edge. When a workpiece is cut using the punch having such a configuration, the bending blade can apply tensile stress to the portion to be cut by the cutting edge. Then, the tensile stress can facilitate propagation of cracks that have been formed in the workpiece by the cutting edge and the die shoulder. This allows the workpiece to be cut by the cutting edge without undergoing compression, and consequently the hole expandability of the punched hole is improved. As a result, it is believed that the occurrence of stretch flange cracking in the sheared edge can be suppressed.
Patent Document 2 discloses a shear blade that includes a main shear blade and an end portion protrusion protruding in the blade advancing direction relative to the main shear blade. When a workpiece sheet is cut using the shear blade having such a configuration, the end portion protrusion can apply tensile stress to the portion to be cut by the main shear blade. As a result, the shear blade of Patent Document 2 achieves advantageous effects similar to those of the punch of Patent Document 1.